Various types of lightweight, high-power batteries are widely used as power sources in vehicle on-board power sources or power sources in personal computers, cell phones and other mobile electronic products, and further applications of such batteries are expected to be ongoingly developed. Lithium secondary batteries are one form of such batteries. A typical lithium ion battery comprises a positive electrode and a negative electrode respectively provided with a layer containing a positive electrode active material and a layer containing a negative electrode active material (active material layers) formed on positive and negative collectors, the positive electrode active material and the negative electrode active material being capable of storing and releasing lithium ions. The lithium ion battery comprises also a non-aqueous electrolyte solution having lithium ion conductivity.
A representative method for forming the active material layers on the collectors of the positive electrode and negative electrodes involves, for instance, applying a composition having a positive or negative active material as a main component (composition for forming an active material layer) and that is prepared, in the form of a paste or slurry, by dispersing the active material in an aqueous solvent or non-aqueous solvent, followed by drying and/or heating to remove the solvent in the active material layer. Besides the solvent active material as a main component, one, two or more types of other materials (typically, conductive materials, binders and the like) are also added to prepare the composition for forming an active material layer.
When the paste or slurry-like composition for forming an active material layer (hereafter “paste for forming an active material layer”) is prepared using a solvent in the form of an organic solvent (non-aqueous solvent), the binder used is a polymer soluble in organic solvents, such as polyvinylidene fluoride (PVDF) or the like. By contrast, when the paste for forming an active material layer is prepared using an aqueous solvent as the solvent, there is preferably used a water-dispersible polymer such as polytetrafluoroethylene (PTFE). Herein, a paste for forming an active material layer wherein the paste uses an aqueous solvent (hereafter “aqueous paste for forming an active material layer”) is advantageous in terms of reducing the overall environmental burden, since such a paste gives rise to little industrial waste, for instance waste water, thanks to not using organic solvents. Equipment and treatment costs associated with that waste can be avoided as a result.
In the preparation of an aqueous paste for forming a positive electrode active material layer, however, there occur exchange reactions between protons and lithium ions at the surface of the positive electrode active material, on account of the high reactivity between the positive electrode active material and water, depending on the nature of the positive electrode active material that is contained in the paste (for instance, a lithium-nickel complex oxide having a composition represented by formula LiNiO2). The pH of the aqueous paste for forming a positive electrode active material layer may rise as a result. When such a high-pH aqueous paste is applied onto the positive electrode collector (for instance, Al foil), compounds exhibiting high electric resistivity (for instance, oxides and hydroxides) form readily on the collector surface, which may result in a greater initial internal resistance of the battery.
Such being the case, Patent document 1 discloses a method that involves forming a beforehand a barrier layer (conductive layer) on the collector surface by applying a paste-like composition comprising a binder, a conductive material and an organic solvent, onto a positive electrode collector surface, and forming then a positive electrode active material layer on the surface of the barrier layer using an aqueous paste for forming a positive electrode active material layer. The method disclosed in Patent document 1 prevents the formation of the above-mentioned resistive compounds through the formation of the barrier layers between the collectors and the active material layers. Patent document 2 is another instance of prior art.
Patent document 1: Japanese Patent Application Laid-open No. 2006-4739
Patent document 2: Japanese Patent No. 3501113
A barrier layer (conductive layer) such as the on disclosed in Patent document 1 requires water resistance, to prevent the generation of high-resistivity compounds on the collector surface, by suppressing direct contact between the collector and the aqueous paste for forming a positive electrode active material layer, and requires also conductivity, so that electric resistance does not rise excessively between the collector and the positive electrode active material layer. To realize good conductivity, for instance, the barrier layer is preferably as thin as possible. However, a thin conventional barrier layer (for instance, the conductive layer disclosed in Patent document 1) tends to have insufficient water resistance.